1. Field of the Invention
The present invention relates to a method and apparatus for calculating physical quantities indicative of internal states (such as charged states) of a secondary (rechargeable) battery to be mounted in a vehicle and an apparatus controlling power voltage in a vehicle, and in particular, the method and apparatus that perform calculation using a plurality of pairs of voltage/current data detected from the battery.
2. Description of the Related Art
On-vehicle battery systems usually use secondary batteries such as lead batteries. In those batteries, it is required to estimate their internal charged states for capacity control and safety control. Conventionally, for those purposes, various types of apparatuses have been provided, which estimate physical quantities indicative of internal charge states of a battery. Such physical quantities are for example the voltage, current, residual capacity, open-circuit voltage and internal resistance of a battery, which will now be called “battery-state quantities” in the following description.
Those estimation apparatuses are characteristic of having a common configuration. That is, many pairs of sampled data of voltage and current are measured as input parameters from a battery, the sampled paired data are used to estimate a relationship between the voltage and the current, and the relationships are for estimating the battery-state quantities.
However, the relationship between the voltage and the current cannot be defined as being simply linear, because there are many physical phenomena that affect the relationship. Such phenomena include the residual capacity, temperature, deteriorated degree, and polarized degree of a battery.
One conventional technique for estimating the relationship between the voltage and the current is to use a regression line. Specifically, a large number of paired data of sampled voltage/current are used to plot voltage (V) and current (I) in a two-dimensional coordinate such that a regression line is produced in the coordinate. This regression line is used to estimate a current value corresponding to a voltage value being inputted or estimate a voltage value corresponding to a current value being inputted. This technique is known as a “regression-line based technique,” which has now been used widely. The slope of a regression line means a value of the internal resistance of a battery.
Other techniques have also been proposed, which are shown in for example Japanese Patent Laid-open Publication Nos. 9-243716 and 2003-249271. These published documents propose how to estimate some battery-state quantities (such as SOC and SOH) serving as output parameters by applying a large number (i.e., a plurality) of pairs of sampled voltage/current data and other battery-state quantities (such as temperature and operating hours) to a neural network. This technique is referred to as a “neural-network based technique.”
However, both conventional regression-line based and neural-network based techniques are still confronted with some difficulties. When the regression-line based technique is used, estimated results of battery-state quantities have large errors. This is due to the fact that the coordinates of a large number of sampled voltage/current pairs are widely dispersed in the two-dimensional coordinate space. The dispersion of the coordinates results in giving a lowered precision to a regression line being estimated.
Meanwhile, for using the foregoing neural-network based technique, amounts of calculation load to a processor increases largely, compared to a case in which regression-line based technique is used. Of course, the processor itself becomes large in size. Nevertheless, this technique has not produced fully satisfactory calculated results yet.
By the way, there is a further difficulty, which should be resolved, between control of power voltage (battery voltage) and changes in the internal charge state of a battery. In the on-vehicle power-supply system, large fluctuations in the power voltage will cause various problems, such as instability of operations of loads, flickering of headlights, and a decrease in the lifetime of a battery. Thus it is strongly demanded to stabilize the power voltage (battery voltage) even when the amount of loads to be powered changes. In conventional vehicles, feedback control is generally done so that a difference between a detected power voltage value and a target voltage value converges to zero. The power voltage controlled on the feedback manner will however cause changes in the internal state of the battery, thus bringing about hunting of the power voltage and/or delay of the control. Therefore the stabilizing control of the power voltage which is in response to sudden changes in the power voltage (, which are due to changes in the electric loads) cannot be performed.